The process control industry employs process variable transmitters to monitor process variables associated with substances such as solids, slurries, liquids, vapors, and gasses in chemical, pulp, petroleum, pharmaceutical, food and other food processing plants. Process variables include pressure, temperature, flow, level, turbidity, density, concentration, chemical composition and other properties. A process variable transmitter can provide an output related to the sensed process variable over a process control loop to a control room, such that the process can be monitored and controlled.
The process control loop can be any configuration of two or more conductors that are capable of passing signals related to process information. For example, the process control loop can be a two-wire, 4-20 mA process control loop. A process variable transmitter coupled to such a process control loop controls the amount of current flowing through the loop such that the current corresponds to the process variable. In some process control loop embodiments, the energization levels can be low enough that even under fault conditions the loop generally will not contain enough electrical energy to generate a spark. This facilitates compliance with the intrinsic safety specification as set forth in the Factory Mutual Approval Standard entitled "Intrinsically Safe Apparatus and Associated Apparatus for Use in Class I, II, and III, Division 1 Hazardous (Classified) Locations," Class Number 3610, published October 1988. Intrinsic safety compliance is particularly relevant in flammable environments. Low power process variable transmitters can operate on such low energy levels that they can receive all required electrical power from a 4-20 mA process control loop. The process control loop may also have digital signals superimposed on the loop according to a process industry standard protocol such as the HART.RTM. digital protocol.
Low Power Time Domain Reflectometry Radar (LPTDRR) instruments have been used recently to measure the level of products (either liquids or solids) in storage vessels. In Time Domain Reflectometry, electromagnetic energy is transmitted from a source, and reflected at a discontinuity. The travel time of the received energy is based on the media through which it travels as well as the distance traveled. One type of LPTDRR is known as Micropower Impulse Radar (MIR), which was developed by the Lawrence Livermore National Laboratory.
Transmitters used in the industrial marketplace see many harsh environmental conditions. Microwave level transmitters can be used in environments with explosive gases. Therefore, in such environments great care must be taken to make the transmitters intrinsically safe, explosion proof, or both. Generally, the microwave termination or antenna must be electrically coupled to a reference voltage such as earth ground so that electrical energy cannot build on the termination and potentially generate a spark. Further, level transmitters can frequently be located considerable distances, reaching 1000 feet or more, from the control room which supplies power for the two-wire loop. The considerable distances between the control room and the transmitter can cause ground loop errors in the 4-20 mA signals. Ground loop error is any undesired voltage drop (noise) between two physically separated connections to ground. Ground loop errors occur because of an impedance between the two connections (typically wire resistance and inductance) and a noise current flowing through the impedance. Additionally, various other industrial devices can create electrical noise at frequencies both higher and lower than the microwave frequency. Such noise can adversely affect transmitter operation.